CATALYSIS
Stable and selective catalysts for propane
dehydrogenation operating at thermodynamic limit
Ali Hussain Motagamwala1,2†, Rawan Almallahi1,2, James Wortman1,2,
Valentina Omoze Igenegbai1,2, Suljo Linic1,2*
Intentional (“on-purpose”) propylene production through nonoxidative propane dehydrogenation (PDH)
holds great promise for meeting the increasing global demand for propylene. For stable performance,
traditional alumina-supported platinum-based catalysts require excess tin and feed dilution with
hydrogen; however, this reduces per-pass propylene conversion and thus lowers catalyst productivity.
We report that silica-supported platinum-tin (Pt 1 Sn 1 ) nanoparticles (<2 nanometers in diameter) can
operate as a PDH catalyst at thermodynamically limited conversion levels, with excellent stability and
selectivity to propylene (>99%). Atomic mixing of Pt and Sn in the precursor is preserved upon
reduction and during catalytic operation. The benign interaction of these nanoparticles with the silicon
dioxide support does not lead to Pt-Sn segregation and formation of a tin oxide phase that can occur
over traditional catalyst supports.
S
hale gas production has led to a switch
in the industrial feedstocks for propyl-
ene, from naphtha to propane ( 1 ). Global
demand for propylene is >100 million
metric tons per year, with ~4% annual
growth ( 2 ). Oxidative propane dehydrogenation
(OPDH) is an emerging technology in which
thermodynamically driven overoxidation of
propane to carbon oxides (COx) is a major
drawback ( 3 , 4 ). For example, state-of-the-art
hexagonal boron nitride OPDH catalysts achieve
a relatively low per-pass propylene yield of <16%
( 5 , 6 ). Additionally, propylene separation from
carbon oxides and other by-products of OPDH
has further limited OPDH application at the
commercial scale ( 7 ).
Nonoxidative propane dehydrogenation (PDH)
produced 13.6 million metric tons of propylene
in 2019, accounting for ~11% of global propylene
production ( 8 ). Commercial PDH processes
utilize either chromium (Cr)–based (Catofin
process) or platinum (Pt)–based catalysts [Oleflex
and steam-activated reforming (STAR) pro-
cesses] ( 9 ). This endothermic reaction operates
at a relatively high temperature (550° to 700°C)
and atmospheric pressure to increase the
equilibrium conversion ( 10 ). The main draw-
backs with Cr-based catalysts are relatively
low selectivity to propylene and high catalyst
deactivation rate due to extensive deposition of
solid carbon (coke) on the catalyst surface. Rapid
catalyst deactivation requires regeneration of
the Catofin catalyst every 12 min ( 11 ).
Relative to the Cr-based catalysts, Pt-based
catalysts in which Pt nanoparticles (NPs)
supported on Al 2 O 3 and modified by the ad-
dition of a post-transition metal such as tin (Sn)
exhibit higher selectivity. Performance depends
critically on forming intermetallic PtSn NPs
that improve propylene selectivity and en-
hance stability by reducing the rate of hydro-
genolysis and coke formation ( 12 ). However,
under PDH reaction conditions, PtSn inter-
metallic NPs on Al 2 O 3 separate into Pt and Sn
atoms and form segregated SnOxand Pt phases
on Al 2 O 3 supports ( 9 ), and this phase-segregated
catalyst deactivates rapidly through coking.
To alleviate this issue, large quantities of Sn
are used in the catalyst preparation (the nominal
atomic loading is Sn:Pt between 4 and 5), which
increases the yield of intermetallic PtSn NPs ( 12 ).
In addition, studies have shown that adding
hydrogen to the reactant feed reduces the rate of
catalyst deactivation ( 13 ). Although this strategy
is implemented commercially ( 10 ), addition of
hydrogen in the feed lowers the thermodynamic
limit on per-pass propane conversion (fig. S1)
and decreases productivity ( 7 ). Even with these
augmentations, to combat deactivation, com-
mercial Pt-based catalysts are operated at low
per-pass propane conversion, and the catalyst
is continuously regenerated with a moving-bed
adiabatic reactor. Thus, catalysts are needed
that can achieve higher per-pass propane con-
version with high propylene selectivity and
elevated catalyst stability compared with the
current processes ( 1 ).
We developed a PDH catalyst that operates
at thermodynamically limited conversion levels
without the addition of hydrogen to the feed
and exhibits excellent stability and selectivity to
propylene (>99%). The catalyst is composed of
silica-supported Pt 1 Sn 1 NPs, synthesized and
pretreated to ensure that the mixing of Pt and
Sn is preserved during catalytic operation. We
synthesized the catalyst by mixing chloropla-
tinic acid (H 2 PtCl 6 ) and tin(II) chloride (SnCl 2 )
in 0.1 M hydrochloric acid solution to form a
heterometallic Pt-Sn coordination complex. We
used this solution to impregnate the SiO 2 sup-port and reduce the catalyst to obtain very small
Pt-Sn NPs. The principle behind catalyst syn-
thesis is to begin with a heterometallic Pt-Sn
coordination complex in which Pt and Sn are in
immediate contact, and this interaction is re-
tained during the reduction and PDH reaction.
This catalyst has three main features: (i) The
PtSn NPs form an atomically mixed Pt-Sn pre-
cursor. (ii) Upon reduction, very small (less
than ~2 nm diameter) intermetallic PtSn NPs
form. (iii) A benign interaction of these NPs
with the SiO 2 support avoids segregation of Pt
and Sn and formation of a SnOxphase. We
compared this Pt 1 Sn 1 /SiO 2 catalyst to a com-
mercial mimic Pt-Sn/g-Al 2 O 3 [the commer-
cially used nominal loading is Pt:Sn = 1:5 ( 9 )]
as well as a Pt/g-Al 2 O 3 catalyst. Catalytic per-
formance was evaluated in a packed-bed reactor
at 580°C under flowing propane, hydrogen, and
an inert gas. Detailed descriptions of the catalyst
synthesis, catalyst characterization, reactor oper-
ation, and analytical methods are available
in the supporting information. Hock and co-
workers used a similar approach to synthe-
size isolated Fe atoms on SiO 2 for PDH by
grafting and reducing the Fe(oCp) 2 complex
[bis(2,4-dimethyl-1,3-pentadienide) iron(II)]
onto a silica support, however, poor propyl-
ene selectivity (~14%) was obtained on this
catalyst ( 14 ).
Data in Fig. 1, A to C, show the performance
of the Pt 1 Sn 1 /SiO 2 catalyst (1 wt % Pt and
0.6 wt % Sn) for propane dehydrogenation
at 580°C, 16 vol % propane (with the remainder
helium) and at a weight hourly space velocity
(WHSV) of 4.7 hours−^1. Under these reaction
conditions, the thermodynamic limit on pro-
pane conversion is 66.5% (fig. S2). The Pt 1 Sn 1 /
SiO 2 catalyst showed near-thermodynamic con-
version of >66% (Fig. 1A, red circles) and >99%
selectivity to propylene (Fig. 1B, red circles). The
Pt/g-Al 2 O 3 (0.5 wt % Pt) catalyst exhibited
relatively poor performance, with only ~15%
initial propane conversion (Fig. 1A, purple cir-
cles) and lower propylene selectivity (Fig. 1B,
purple circles). Similarly, the commercial mimic
Pt-Sn/g-Al 2 O 3 catalysts showed modest activity,
with initial propane conversion of 22% (Fig. 1A,
black triangles) and comparatively lower pro-
pylene selectivity (Fig. 1B, black triangles). Note
that the active phase in these commercial mimics
is Pt 3 Sn, with the excess Sn present as the SnOx
layers on the alumina support ( 12 ).
Addition of hydrogen to the feed stream
improved the activity and stability of the Pt/
g-Al 2 O 3 and the commercial mimic Pt-Sn/
g-Al 2 O 3 catalysts (Fig. 1, A and B, blue squares
and green triangles, respectively), albeit to lev-
els much lower than those achieved by Pt 1 Sn 1 /
SiO 2. Addition of hydrogen did not compromise
the performance of Pt 1 Sn 1 /SiO 2 (Fig. 1, A and B,
purple diamonds). Data in Fig. 1C show the
propylene yield (product of the selectivity and
conversion) for the tested catalysts, demonstratingSCIENCEsciencemag.org 9JULY2021•VOL 373 ISSUE 6551 217
(^1) Department of Chemical Engineering, University of Michigan,
Ann Arbor, MI, USA.^2 Catalysis Science and Technology
Institute, University of Michigan, Ann Arbor, MI, USA.
*Corresponding author. Email: [email protected]
†Present address: Shell Chemical Company, Shell Technology
Center Houston, 3333 Highway 6 South, Houston, TX 77082, USA.
RESEARCH | REPORTS